1. Field of the Invention
The present invention generally relates to a radiation detector, for example, applied to a positron emission tomography (PET) device, a single photon emission computed tomography (SPECT) device, or other medical diagnostic devices, in which the device detects a radioactive ray (gamma ray) discharged from radioactive isotopes (RIs) that are administered to a to-be-tested object and accumulated in a target site, so as to obtain an RI distribution tomogram of the target site.
2. Description of Related Art
The radiation detector includes scintillators that become luminescent by the incident gamma ray discharged from the to-be-tested object, and photomultipliers converting the luminescence of the scintillators to a pulsed electric signal. For the radiation detector of the prior art, the scintillators one-by-one correspond to the photomultipliers. However, the following method is being adopted recently, in which the photomultipliers with the number less than that of the scintillators are combined with a plurality of scintillators. And according to the power ratio of the photomultipliers, the incident position of the gamma ray is determined, so as to improve the resolution (for example, refer to patent document 1).
FIG. 4 is a cross sectional view in the X direction (front view) obtained by viewing a conventional radiation detector 50 from the Y direction. When the radiation detector is an isotropic voxel detector, a cross sectional view in the Y direction (side view) of the conventional radiation detector 50, viewing from the X direction, also has the same shape as that of FIG. 4. The radiation detector 50 includes a scintillator array 12, which is divided by appropriately inserting a light reflective material 13 into 36 scintillators 11 that are two dimensionally and compactly arranged in a manner of six scintillators in the X direction and six scintillators in the Y direction; a light guide 14, which is optically combined with the scintillator array 12 and is divided into a plurality of small blocks, and includes embedded lattice frames combined with a light reflective material 15; and four photomultipliers 201, 202, 203, and 204 optically combined with the light guide 14. Additionally, in FIG. 8, only the photomultipliers 201 and 202 are shown, and the scintillators 11, for example, apply Bi4Ge3O12 (BGO), Gd2SiO5:Ce (GSO), Lu2SiO5:Ce (LSO), LuYSiO5:Ce (LYSO), LaBr3:Ce, LaCl3:Ce, NaI, CsI:Na, BaF2, CsF, PbWO4, and other inorganic crystals.
If the gamma ray is incident on any one of the six scintillators 11 arranged in the X direction, the gamma ray is converted to visible light. The light is guided to the photomultipliers 201-204 through the optically combined light guide 14. At this time, the position, length, and angle of each light reflective material 15 in the light guide 14 are adjusted, such that the power ratio of the photomultiplier 201 (203) to the photomultiplier 202 (204) arranged in the X direction is changed according to a fixed ratio.
Particularly, when the power of the photomultiplier 201 is set to P1, the power of the photomultiplier 202 is set to P2, the power of the photomultiplier 203 is set to P3, and the power of the photomultiplier 204 is set to P4, and the position and the length of the light reflective material 15 are set, such that a calculated value {(P1+P3)−(P2+P4)}/(P1+P2+P3+P4) representing a position in the X direction is changed in accordance with the position of each scintillator 111 at a fixed ratio.
In another aspect, for the six scintillators arranged in the Y direction, similarly the light is guided to the photomultipliers 201˜204 through the optically combined light guide 14. That is, the position and the length of each light reflective material 15 in the light guide 14 are set, and the angle is adjusted under an inclined condition, such that the power ratio of the photomultiplier 201 (202) to the photomultiplier 203 (204) arranged in the Y direction is changed at a fixed ratio.
That is, the position and length of the light reflective material 15 are set, such that the calculated value {(P1+P2)−(P3+P4)}/(P1+P2+P3+P4) representing a position in the Y direction is changed at a fixed ratio in accordance with the position of each scintillator.
In the conventional radiation detector 50, the light reflective material 13 between the scintillators 11 and the light reflective material 15 of the light guide 14 may use a silica and titania multi-layer film with a polyester film base material. The reflection efficiency of the multi-layer film is quite high, so the multi-layer film is used as a light reflective element. However, strictly speaking, a part of the light may be transmitted because of the incident angle of the light. Therefore, the shape and disposition of the light reflective material 13 and the light reflective material 15 are determined according to the transmission part of the light.
In addition, the scintillator array 12 is adhered to the light guide 14 by a coupling adhesive to form a coupling adhesive layer 16, and the light guide 14 is also adhered to the photomultipliers 201˜204 by the coupling adhesive to form a coupling adhesive layer 17. Except for the surfaces optically combined with the photomultipliers 201˜204, the peripheral surfaces which are not opposite to each scintillator 11 are covered by the light reflective material. At this time, the light reflective material mainly uses a polytetrafluoroethylene (PTFE) adhesive tape.
FIG. 5 is a block diagram of the structure of a position operating circuit of the radiation detector. The position operating circuit is formed by adders 21, 22, 23, 24, and position determining circuits 25 and 26. As shown in FIG. 5, in order to detect the incident position of the gamma ray in the X direction, the power P1 of the photomultiplier 201 and the power P3 of the photomultiplier 203 are input to the adder 21, and the power P2 of the photomuitiplier 202 and the power P4 of the photomultiplier 204 are input to the adder 22. The added powers (P1+P2) and (P3+P4) output by the two adders 21 and 22 are input to the position determining circuit 25, and the incident position of the gamma ray in the X direction is found out according to the two added powers.
Similarly, in order to detect the incident position of the gamma ray in the Y direction, the power P1 of the photomultiplier 201 and the power P2 of the photomultiplier 202 are input to the adder 23, and the power P3 of the photomultiplier 203 and the power P4 of the photomultiplier 204 are input to the adder 24. The added powers (P1+P2) and (P3+P4) output by the two adders 23 and 24 are input to the position determining circuit 26, and the incident position of the gamma ray in the Y direction is found out according to the two added powers.
In addition, the calculated value (P1+P2+P3+P4) represents the energy relative to the event, and is represented by an energy spectrum as shown in FIG. 6.
For the result calculated with the previous method, it is represented by a position coding map as shown in FIG. 7 according to the positions of the gamma ray incident on the scintillators, and the result also represents the determined information of each position.
In another aspect, methods for improving the spatial resolution by realizing block detectors having the depth of interaction (DOI) information are proposed, for example, a method of compactly disposing several layers of the scintillator arrays respectively formed by materials with different luminescence decay time (for example, please refer to non patent document 1), or a method of disposing each scintillator array in a manner of being spaced by a half pitch (for example, please refer to non patent document 2) and the like.
In a plurality of the examples in the prior art, the photomultiplier is used as a light receiving element receiving the light emitted by any scintillator. For the radiation detector 60 as shown in FIG. 8, semiconductor light receiving elements called avalanche photodiodes 301˜304 are also used recently. The avalanche photodiodes are used in an avalanche state by applying a high electric field into a silicon depletion layer, so as to perform the signal amplification. A signal amplification degree of the avalanche photodiode is 50˜100 times, which is smaller than the amplification degree of the photomultiplier of 105˜106 times. However, the avalanche photodiode can be applied by using a low noise amplifier or in a low temperature environment. As the avalanche is generated in a thinner silicon depletion layer, compared with the photomultiplier, the avalanche photodiode serving as the light receiving element is quite thin, such that under a situation that the space is limited, it is extremely effective to a detector in the PET device.
Patent Document 1: Japanese Patent Publication Number 2004-354343
Non Patent Document 1: S. Yamamoto and H. Ishibashi, A GSO depth of interaction detector for PET, IEEE Trans. Nucl. Sci., 45:1078-1082, 1998.
Non Patent Document 2: H. Liu, T. Omura, M. Watanabe, et al., Development of a depth of interaction detector for g-rays, Nucl. Instr. Meth., Physics Research A 459:182-190, 2001.
However, the radiation detector of the examples in the prior art has the following problems.
In the radiation detector as shown in FIG. 4, the photomultipliers are used as light receiving elements receiving the light emitted by the scintillators. The photomultipliers are much larger than the scintillator array. For the detector in the PET device, there is a big problem under the situation that the space is limited. In addition, a plurality of electrodes or dynodes is complicatedly disposed in the photomultiplier, which cannot be realized at a low cost.
In another aspect, in the radiation detector as shown in FIG. 8, the avalanche photodiodes are used as light receiving elements receiving the light emitted by the scintillators. The avalanche photodiodes are quite thin and have a simple structure, so they can be fabricated to be compact. However, the signal amplification degree of the avalanche photodiodes is 50-100 times, which is smaller than the amplification degree of the photomultipliers of 105˜106 times. Therefore, the avalanche photodiodes can be applied by using an expensive low noise amplifier and a dedicated temperature adjusting mechanism in the low temperature environment. In addition, the luminescence wavelength of the scintillator LaBr3:Ce or LaCl3:Ce with high performance, high luminescence, and high speed is 300˜400 nm, which is a low wavelength band, and the quantum efficiency of the avalanche photodiodes in the wavelength band of 300˜400 nm is 40˜60%, such that the efficiency is poor.